For decades public health officials thought the Zika virus caused only relatively mild illnesses in people. Since the start of an outbreak in Brazil in 2015, however, it has become horribly clear that the virus can pass from pregnant women to their fetuses, with devastating consequences. The virus kills some of the unborn children and leaves others with severe brain damage, including smaller than normal heads (a condition called microcephaly). How the virus reaches the fetus is a mystery because to get there, it must cross the placenta, a pancake-shaped organ that connects the developing infant to the mother and that manages to block transmission of other closely related mosquito-borne viruses, such as dengue and yellow fever, from mother to baby.